1. Field of the Invention
The present invention relates to a method of fabricating semiconductor devices, and more particularly, to a method of fabricating a gate for repairing damage to a gate oxide layer.
2. Description of the Related Art
With a demand for high-speed operation of semiconductor devices, a material having a higher conductance is used as an electrode which contacts a gate, a drain or a source. For example, a material such as silicide or metal is used as a gate or electrode as shown in U.S. Pat. No. 5,814,537 to Maa, et. al., entitled "Method of Forming Transistor Electrodes from Directionally Deposited Silicide", filed on Sep. 29, 1998, or U.S. Pat. No. 5,194,403 to Delage, et. al., entitled "Method for the Making of the Electrode Metalizations of a Transistor", filed on Apr. 16, 1993. Also, tungsten silicide WSi.sub.x can be used as a gate as in U.S. Pat. No. 5,804,499 to Dehm, et. al., entitled "Prevention of Abnormal Wsi.sub.x Oxidation by in-situ Amorphous Silicon Deposition", filed on Sep. 8, 1998. Also, this patent discloses protection of a tungsten silicide layer using an amorphous silicon layer to prevent oxidation of tungsten silicide.
Meanwhile, in order to form a gate, a process for patterning a conductive layer at a required scale is performed after a gate oxide layer and the conductive layer are deposited on a semiconductor substrate. In this patterning process, the gate oxide layer below the conductive layer can be damaged. Particularly, the edge of the gate oxide layer, that is, a portion adjacent to the sidewall of the patterned conductive layer, is damaged more than other portions. This damage can cause a degradation in the characteristics of a transistor, so that a thermal treatment process for repairing the damaged gate oxide layer is required.
However, the thermal treatment may cause a defect within a gate. For example, as shown in FIG. 1, a defect such as a cavity or void 37 may be formed within a gate which comprises polycrystalline silicon layer 31/tungsten silicide layer 35.
To be more specific, a gate oxide layer 20 is deposited on a semiconductor substrate 10, and the polycrystalline silicon layer 31 and the tungsten silicide layer 35 are sequentially formed and patterned by dry etching. At this time, the gate oxide layer 20 under the polycrystalline silicon layer 31 is eroded and damaged by the dry etching. In order to repair this damage to the gate oxide layer 20, a thermal treatment with an oxidation atmosphere is performed. During the thermal treatment, the sidewall surfaces of the patterned polycrystalline silicon layer 31 and tungsten silicide layer 35 can be oxidized, resulting in a silicon oxide layer 20'. Accordingly, the damage to the gate oxide layer 20 is repaired, particularly damage to the edge thereof adjacent to the sidewall of the patterned polycrystalline silicon layer 31.
In the tungsten silicide layer 35, a supply of silicon in this thermal treatment is accomplished by consumption of excessive silicon contained in the tungsten silicide layer 35 itself. However, the amount of the excessive silicon is limited, so that the tungsten silicide layer 35 acts as a silicon supply source only in the early stage of thermal treatment. As thermal treatment progresses, silicon required by oxidation is provided from the polycrystalline silicon layer 31 below the tungsten silicide layer 35.
That is, silicon contained in the polycrystalline silicon layer 31 moves into the tungsten silicide layer 35 or to the surface thereof due to diffusion or the like, and is consumed for oxidation. This movement of silicon may generate cavity 37 within the polycrystalline silicon layer 31. Consequently, the generation of the cavity 37 deteriorates the operational characteristics of a transistor.
In particular, when dichlorosilane SiH.sub.2 Cl.sub.2 (hereinafter referred to as "DCS") is used as a source gas used for depositing the tungsten silicide layer 35 in order to reduce the content of fluorine (F) in the tungsten silicide layer 35, the generation of the cavity 37 becomes serious. When the tungsten silicide layer is formed of DCS or the like, a very small amount of chlorine (Cl) remains within the tungsten silicide layer 35. Chlorine (Cl) helps diffuse silicon, thus increasing the mobility of silicon from the polycrystalline silicon layer 31.